The present invention relates to an output circuit, and more particularly, to a convergence output circuit for decreasing power consumption in a projection television.
Generally, a projection-type color picture tube enlarges and projects an image onto a screen by means of three projection tubes each emitting one of three primary colors. The incident angle from a projection tube to the screen is different for each projection tube. The difference in incident angle from each projection tube results in color deviation of the image reproduced on the screen. The superposition of these three colors (hereinafter referred to as convergence) is controlled by varying the amplitude and shape of a convergence compensating waveform which is generated in synchronization with horizontal and vertical scanning periods of a video signal.
Conventional convergence output circuits include an amplifier U1, drive transistors Q1 and Q2, resistors R1-R4 and a convergence coil L, as shown in FIG. 1. The amplifier U1 compares an input signal V.sub.IN, shown in FIG. 2A, applied to a non-inverting input of the amplifier U1 with a feedback signal V.sub.f applied to an inverting input of the amplifier U1, and outputs first and second output signals OS1 and OS2.
Drive transistor Q1 is a PNP bipolar transistor which includes an emitter connected to a first power source voltage (+Vcc) via a current-limiting resistor R1 and a base which receives the first output signal OS1 from the amplifier U1. Drive transistor Q2 is an NPN bipolar transistor which includes an emitter connected to a second power source voltage (-Vcc) via a current-limiting transistor R2 and a base which receives the second output signal OS2 from the amplifier U1. The collectors of the bipolar transistors Q1 and Q2 are commonly connected, with the connection point being coupled to one end of the convergence coil L via a protection resistor R3. The other end of the convergence coil L is grounded via a feedback resistor R4.
The voltage across resistor R4 is applied to the inverting input of the amplifier U1 as a feedback signal V.sub.f. That is, in the conventional convergence output circuit, the emitter currents of transistors Q1 and Q2 are controlled by the voltage difference between the input signal V.sub.IN and the feedback signal V.sub.f, both of which are applied to the amplifier U1, respectively, so that a current I.sub.L having a waveform shown in FIG. 2B is supplied to the convergence coil L. The passage of current I.sub.L through the convergence coil L deflects the electron beams so that the convergence of the three primary colors can be controlled. The voltage induced by the convergence coil L is represented by a waveform shown in FIG. 2C, wherein one period is divided into a blanking period T.sub.1 and a scanning period T.sub.2. Voltages V.sub.T1 and V.sub.T2 appearing during periods T.sub.1 and T.sub.2, respectively, can be expressed as follows: ##EQU1## Since T.sub.2 is much larger than T.sub.1, V.sub.T1 is much greater than than V.sub.T2 and V.sub.T2 can be virtually ignored with respect to V.sub.T1.
In order to allow transistors Q.sub.1 and Q.sub.2 to control the output current I.sub.L sufficiently, the first and second power source voltages +Vcc and -Vcc should be set to values such that a voltage higher than V.sub.T1 is applied as the power source voltage of the convergence output circuit. That is, the following relationships should be satisfied: EQU +Vcc&gt;V.sub.T1(max) +(R1+R3+R4)I.sub.L(max) +V.sub.Q1(sat) ( 3)
and EQU -Vcc=-(V.sub.T1(max) +(R1+R3+R4)I.sub.L(max) +V.sub.Q2(sat)( 4)
where ##EQU2##
Since the current I.sub.L flowing through the convergence coil is supplied by the power source +Vcc or -Vcc, and the voltage Vcc is significantly higher than V.sub.T1, the power consumption PW in the convergence output circuit is as follows: EQU PW=.+-.Vcc.times.I.sub.L. (6)
The above-obtained power consumption is very large, and therefore, a problem results in that a large power supply circuit becomes necessary.